Game ball monitoring method and apparatus

ABSTRACT

A method and apparatus for determining in a ball game (for example, tennis) the position, relative to a service reference lines ( 12 ), of the first contact of the ball ( 14 ) with the playing surface. In play, the ball passes through a collimated light beam to intercept light directed at a light sensing array, which comprises many closely spaced sensors ( 46 ) arranged in column and row formation. A signal storage means includes one storage device for each of the sensors, so that the position of the ball may be tracked in the storage means as it passes through the light beam. The said position is determined by tracking in the storage means the front of the ball ( 14 F), and for each sensed position of the ball front determining the then height (H) of the top of the ball, at a position displaced rearwardly one half the ball diameter. The detected height of the top of the ball is compared for each position of the ball front with a ball reference height to ascertain the said ball position at the moment the height of the top of the ball equates with the ball reference height. Alternatively, real-time comparisons of detected and reference ball heights may be made by a computer operating on the interrupt principle, thus obviating the need for the storage devices in the making of comparisons.

[0001] This invention relates to a method and apparatus for monitoring in a ball game the position relative to a reference line at which a game ball strikes a playing surface. The invention has particular reference to the game of tennis, though it is not restricted to that game.

[0002] In what follows hereafter, the present invention will be described, for convenience only and by way of example, in the context of that game.

[0003] In the game of tennis, it is important to be able to determine with the greatest accuracy and consistency the position at which a ball on being served first strikes the playing surface relative to the ‘service line’. That line defines the distant boundary of the service area in which the served tennis ball must first strike the playing surface. A service which results in the ball first striking the playing surface beyond that service line is a faulty (invalid) serve.

[0004] There have been various proposals to provide an apparatus for judging whether a served ball has landed on the playing surface within or beyond the service line.

[0005] In my British Patent No. 2,292,218, I have described and claimed one system for ‘service line judging’; the reader is hereby referred to that Patent for further information concerning that system.

[0006] The present invention represents a further development of the system described in that Patent, and has various features which are common to that prior art system.

[0007] According to one aspect of the present invention there is provided a method of monitoring in a ball game the position, relative to a reference line extending linearly between near and far ends thereof, of the first contact of a game ball with a playing surface carrying said reference line, which method includes the following steps:

[0008] (a) from said near end of the reference line directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of a game ball through the radiation when in play near the reference line;

[0009] (b) at said far end of the reference line sensing the game ball's presence in the radiation by detecting the shadow cast at that end by the game ball;

[0010] (c) when the game ball is in flight through said field detecting from said shadow the passage of the front of the game ball through successive closely-spaced positions in the direction of flight of the game ball;

[0011] (d) for each such closely-spaced position, determining the height of the top of the game ball at a position spaced one half the diameter of the game ball rearwardly of the front of the game ball;

[0012] (e) for each such closely-spaced position, comparing the detected height of the top of the game ball with a game ball diameter reference height stored in a storage means; and

[0013] (f) determining as the position of first contact of the game ball with the playing surface the game ball position at which the reducing-detected height of the top of the game ball has become equal to the game ball diameter reference height stored in said storage means.

[0014] Preferably, the stored game ball diameter reference height is determined by detecting the height of the shadow cast at said far end by the game ball when lying stationary on the playing surface at the reference line.

[0015] In one embodiment of the present invention, said method includes the following steps:

[0016] (a) from said near end of the reference line directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of the game ball through the radiation when in play near the reference line;

[0017] (b) at the far end of the reference line, receiving at closely spaced locations in said field respective elements of said radiation, which locations are disposed uniformly in vertical column and horizontal row formation;

[0018] (c) converting the respective received radiation elements into corresponding electrical signals;

[0019] (d) storing the respective electrical signals in respective signal storage elements of a storage means for retention therein until set by the passage of the game ball through the radiation, each said storage element when storing a said electrical signal representing a radiation element that has not been interrupted by the game ball, and each said storage element when not so storing a said electrical signal representing a radiation element which has been interrupted by the game ball;

[0020] (e) defining in said storage means a game ball diameter reference height;

[0021] (f) tracking in said storage means the front of the game ball as it moves forwardly through the radiation, by following the first shading by the game ball of sensors in successive next columns of sensors in the direction of game ball flight;

[0022] (g) for each of successive positions of the front of the game ball, determining in said storage means the height of the top of the game ball at a position disposed one half of the diameter of the game ball rearwardly from the front of the ball—by determining in the column of sensors at said rearwardly disposed position the height of the uppermost sensor not receiving radiation;

[0023] (h) determining the instant at which the reducing detected height of the top of the game ball equates to the ball diameter reference height; and

[0024] (i) determining the position of the game ball at that instant, which position is the position of first contact of the game ball with the playing surface relative to said reference line.

[0025] Preferably, the stored game ball diameter reference height is defined by determining the lowest row of sensors in which all of the sensors in the row continue to receive radiation when the game ball lies stationary on the playing surface at the reference line.

[0026] According to a second aspect of the present invention there is provided an apparatus for monitoring in a ball game the position, relative to a reference line extending linearly between near and far ends thereof, of the first contact of a game ball with a playing surface carrying said reference line, which apparatus comprises:

[0027] (a) at said near end of the reference line, radiation emitting means for directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of the game ball through the radiation when in play near the reference line;

[0028] (b) at said far end of the reference line, radiation receiving means comprising a plurality of radiation sensors for receiving at closely spaced locations in said field respective elements of said radiation, which locations are disposed uniformly in vertical column and horizontal row formation, which radiation receiving means is arranged to sense the game ball's presence in the radiation by the shadow cast at said far end by the game ball;

[0029] (c) means for detecting when the game ball is in flight through said field the passage of the front of the game ball through successive closely-spaced positions in the direction of flight of the game ball;

[0030] (d) means for determining, for each such closely-spaced position, the height of the top of the game ball at a position spaced one half the diameter of the game ball rearwardly of the front of the game ball;

[0031] (e) means for comparing, for each such closely-spaced position, the detected height of the top of the game ball with a stored game ball diameter reference height; and

[0032] (f) means for determining as the position of first contact of the game ball with the playing surface the game ball position at which the reducing detected height of the top of the game ball has become equal to the stored game ball diameter reference height.

[0033] Preferably, there is provided means for determining the game ball diameter reference height from said shadow when the game ball lies stationary on the playing surface at the reference line.

[0034] In one embodiment of the present invention, said apparatus comprises:

[0035] (a) at said near end of the reference line, radiation emitting means for directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of the game ball through the radiation when in play near the reference line;

[0036] (b) at the far end of the reference line, radiation receiving means comprising a plurality of radiation sensors for receiving at closely spaced locations in said field respective elements of said radiation, which locations are disposed uniformly in vertical column and horizontal row formation;

[0037] (c) converting means for converting the respective received radiation elements into corresponding electrical signals;

[0038] (d) electrical signal storage means for storing the respective electrical signals in respective signal storage elements of the storage means for retention therein until set by the passage of the game ball through the radiation, each said storage element when storing a said electrical signal representing a radiation element that has not been interrupted by the game ball, and each said storage element when not so storing a said electrical signal representing a radiation element which has been interrupted by the game ball;

[0039] (e) means for determining as a game ball diameter reference height, the lowest row of sensors in which all sensors in the row continue to receive radiation when the game ball lies stationary on the reference line;

[0040] (f) means for tracking the front of the game ball as it moves forwardly through the radiation, by reference to the first shading by the game ball of sensors in successive next columns of sensors;

[0041] (g) means for determining for each of successive positions of the front of the game ball the height of the top of the game ball at a position disposed one half of the diameter of the game ball rearwardly from the front of the game ball;

[0042] (h) means for determining the instant at which the reducing detected height of the top of the game ball equates to the game ball diameter reference height; and

[0043] (i) means for determining the position of the game ball at that instant, which position is the position of first contact of the game ball with the playing surface relative to said reference line.

[0044] Said radiation may comprise visible light, or infra-red radiation, or any other convenient and suitable form of radiation.

[0045] Other features of the present invention will appear from a reading of the description that follows hereafter and of the claims appended at the end of that description.

[0046] One apparatus (and various modifications thereof) for use in connection with the game of tennis and embodying the present invention will now be described by way of example and with reference to the accompanying diagrammatic drawings. In those drawings:

[0047]FIG. 1 shows in side view a representation of a tennis court playing surface with one tennis ball sitting on the playing surface, and the same ball in flight towards the playing surface after being served;

[0048]FIG. 2 shows pictorially and schematically the principal components of the apparatus and their respective inter-relationships and inter-connections;

[0049]FIG. 3 shows the components forming a light source as used in the apparatus;

[0050]FIG. 4 shows a view, in the direction of the arrow IV of FIG. 2, of the light receiving face of a photo-sensor array forming part of the apparatus, showing the path of a tennis ball in terms of photo-sensors that are ‘unshaded’ and ‘shaded’ respectively during the passage of the ball;

[0051]FIG. 5 shows the component parts of a single light sensor incorporated in the light sensor array;

[0052]FIG. 6 shows an electric circuit diagram of a bistable circuit module-for storing the status (‘shaded’ or ‘unshaded’) of an associated photo-sensor; and

[0053]FIG. 7 shows an electric circuit diagram showing the manner of interconnection of two columns of bistable circuit modules which are associated with two specific columns of sensors which are spaced one half a tennis ball diameter apart.

[0054] In the respective figures, parts which appear in more than one figure or correspond to parts in another figure bear the same reference numeral in all of the respective figures, unless otherwise indicated.

[0055] Referring now to the drawings, the concept underlying the present method and apparatus will be described and explained with reference to FIG. 1.

[0056] In that Figure, a tennis court playing surface 10, indicated in vertical section, incorporates a service line 12 which extends across the tennis court in a direction normal to the plane of the figure.

[0057] For a tennis ball 14 served from the right hand side of the figure, the left hand end 12A of the service line 12 represents the distant boundary for touch-down of a properly served ball, and will be referred to as the ‘service reference line’ 12A. A ball touching down at that reference line or to the right of it constitutes a valid serve.

[0058] On the other hand, a ball touching down to the left of that service reference line 12A without touching the service line constitutes a faulty (invalid) serve.

[0059] However, the rules of the International Tennis Federation allow a tennis ball to be judged as properly served if some part of the ball touches the service line even if the ball has first touched-down at a position to the left (that is beyond) the service reference line 12A.

[0060] Deformation of the ball due to its striking the playing surface at an acute angle with considerable force can cause the moving ball to momentarily squash back on to the service line even though the point of first touch-down lies to the left of the service reference line 12A. Such a ball is deemed under the above-mentioned rules to have been properly served.

[0061] The extent to which squash-back of a ball occurs depends on the angle at which it approaches the playing surface, being greater for greater angles relative to the playing surface.

[0062] The tennis ball is also shown in the FIG. 1, at reference 16, in its normal unstressed condition and sitting on the playing surface with its lowermost part 16B (i.e. the bottom of the ball) in contact with the playing surface 10 at the service reference line 12A.

[0063] The height above the playing surface 10 of the uppermost part 16T (i.e. the top) of the ball when in its normal unstressed condition is indicated by chain-dotted ‘ball diameter reference line’ 18 spaced above the playing surface at a height equal to the diameter ‘D’ of the unstressed tennis ball.

[0064] The same ball when in flight from right to left during a service is represented at reference 14. The top 14T of the ball, when in that in-flight position, is situated at a height ‘H’ above the playing surface 10 and service reference line 12A, where ‘H’ exceeds the height of the ball diameter reference line 18 by an excess amount ‘E’. Touch-down of the ball on the playing surface 10 will occur when ‘E’ falls to zero (‘H’ then falling to ‘D’), that is when the top 14T of the ball falls to the level of the ball diameter reference line 18.

[0065] According to the present invention, touch-down is determined by sensing periodically the horizontal position of the front 14F of the ball, and for each position so sensed determining at a second position—spaced one half the diameter of the ball to the right of that position—the height ‘H’ of the top 14T of the ball above the playing surface 10. Touch-down occurs when the sensed height ‘H’ falls to the height ‘D’ of the ball diameter reference line 16. The touch-down position is then given by the said second position at which ‘H’ has fallen to ‘D’.

[0066] For the ball in flight to be judged as having touched down on or before the service reference line 12A, the excess ‘E’ by which ‘H’ exceeds ‘D’ must have fallen to zero value by the time that the horizontal distance ‘X’ (between the front 14F of the ball and the service reference line 12A) has fallen to zero value.

[0067] To enable a ball to be judged ‘in’ (i.e.within the service court) when squash-back brings the ball momentarily into contact with the service line 12 despite a late touch-down (i.e. beyond the service reference line 12A), appropriate empirical compensation is made in dependence upon the angle at which the ball approaches the service line, such compensation being greater for larger angles of approach.

[0068] Referring now to the FIG. 2, the apparatus there shown comprises on one side of a tennis court (not indicated) and at one end of a service line 28 (equivalent to the service line 12 of FIG. 1)—a light source 30, and at the other side of the tennis court and at the far end of the service line 28—a light sensor array 32. Both the light source and sensor array have adjustable feet 34,36 for enabling proper alignment of the light source and sensor array across the width of the tennis court.

[0069] As shown in FIG. 3, the light source 30 comprises a light emitter 38 in the form of a collimated laser, a collimating lens system 40 for projecting a homogeneous, collimated beam of light 42 of uniform light intensity across the tennis court to the sensor array 32. That light beam has a transverse cross section of rectangular shape for illuminating uniformly the whole of the front face 44 of the sensor array 32. That front face 44 is also shown in FIG. 4.

[0070] As shown in FIG. 4, the sensor array 32 comprises a uniform, row and column matrix of light (photo) sensors 46, made up in this example of some sixteen sensors in each of sixty vertical columns. Those sensors have a small pitch of, for example, one to three millimetres. The pitch in the horizontal rows may conveniently be greater than the pitch in the vertical columns.

[0071] As shown in FIG. 5, each light sensor 46 comprises a photo-diode 48 which is optionally disposed at the output end of an optical fibre ‘light pipe’ 50. Where provided, the free, input ends 52 of the light pipes are secured together with optical insulation therebetween to form the light sensitive front face 44 of the sensor array.

[0072] As will be seen in FIG. 2, the sensor array 32 has an electrical output channel 56 for feeding output signals derived from the respective photo-diodes 48 to an electric signal storage array 58 which comprises a plurality of bi-stable circuit modules 60, one for each of the respective light sensors 46.

[0073] As shown in FIG. 6, each bistable circuit module 60 includes a high input-impedance amplifier 62 having (a) an input circuit 64 supplied by the associated photo-diode 48, (b) a first output circuit 66 which supplies an OR gating circuit 68, and (c) a second output circuit 70 which controls and determines the status of an electronic bistable device 72 in accordance with the ‘shaded’ or ‘unshaded’ state of the associated photo-diode 48.

[0074] That bistable device has an input circuit 74 for receiving a trigger signal from an OR gate 68 associated with a sensor column 106 disposed to the left at a distance of one half of a ball diameter, and an output circuit 76 for transmitting to a tristate electronic device 78 a signal representing the current status of the bistable device 72. The status of the bistable device 72 thus follows that of the associated photo-diode 48, but on receipt of an input trigger signal 80 it retains the status that it held at the moment the input trigger signal 80 was received.

[0075] The status of the bi-stable device 72 is transmitted over its output circuit 76 to an electronic tristate device 78, which has an input circuit 82 for receiving ‘read’/‘write’ signals from a computer 84, and an output circuit 86 for delivering to the computer, in response to ‘read’ signals, signals representative of the status of the bistable device 72 at the time of receipt of the trigger signal 80.

[0076] The input and output circuits 82 and 86 of the tristate devices of the respective bistable circuit modules 60 form and input/output signal transmission highway 88 interconnecting those modules with the computer 84.

[0077] The computer 84 has an associated memory 90 into which the states of the respective tristate devices 78 may be transferred for retention there (until subsequently erased) and analysis. The computer has also an associated monitor device 92, upon the screen 94 of which the computer can display the respective states of the tristate devices 78, as a series of bright areas on a dark background. Those bright areas are created in response to the absence of light at the respective associated photo-diodes 48. The screen of the monitor is preferably of sufficient size to show the state of all the sensors in the sensor array.

[0078] In the absence of any obstructing object between the light source 30 and the sensor array 32, all of the light sensors in the array 32 are illuminated uniformly by respective elemental areas of the light beam 42, so that the corresponding bistable circuit modules 60 all indicate the illuminated condition (‘unshaded’) of the sensors, and the whole of the monitor screen remains dark.

[0079] When a tennis ball enters the field between the light source 30 and the sensor array 32, the sensors 46 directly behind the tennis ball lie in the shadow of the ball, and so receive no light from the light source. Thus, in turn the associated bistable circuit modules 60 record that new condition in which the sensors disposed behind the ball are shaded and therefore not illuminated. In that condition, the monitor screen 92 shows bright illuminated areas corresponding to the shaded sensors. In this way, a bright image of the passage of the tennis ball is portrayed on the monitor screen.

[0080] Referring now to the schematic diagram of FIG. 7, there is shown there the circuitry for two columns 100, 102 of sensors which are spaced apart by one half the ball diameter ‘D’. The presence of intervening columns of sensors is merely indicated by various spaced chain-dotted vertical lines 104. Other columns of sensors disposed to either side of the columns 100 and 102 are indicated by the vertical lines 106 and 108 respectively.

[0081] Consider now the operation of the apparatus for a tennis ball in flight, having been served from the right—particularly in relation to the part of the circuitry shown in FIG. 7, taken in association with that shown in FIG. 6.

[0082] The ball intercepts first the light illuminating the right hand sensor column 102, and then in succession the light illuminating the respective other sensor columns to the left.

[0083] Consider first the moment at which the ball is about to intercept the light reaching the sensors in the column 100. Before that moment the amplifier output circuits 66 of all of the bistable modules 60 of that sensor column indicate that they are all receiving light from the light source 30, so that the associated OR gate 64 has no output signal 80.

[0084] The output circuits 70 of all of those amplifiers 62 cause the associated bistable devices 72 to represent the ‘unshaded’0 status of the sensors of that column 100.

[0085] Shading now of any one of the sensors 46 of the column 100 by further movement of the ball to the left causes the associated OR gate 64 to receive at least one ‘shaded’ input signal, thus causing the OR gate to emit a trigger signal 80 to the input circuits 74 of all of the bistable circuit modules 60 associated with the sensor column 102 which is situated one half the ball diameter to rear. Those bistable circuit modules 60 thereupon set themselves in the state in which they existed at the moment of receipt of those trigger signals 80, and accordingly set the tristate devices in corresponding ‘shaded’ or ‘unshaded’ states.

[0086] Addressed ‘read’ signals now transmitted by the computer 84 to the respective tristate devices 78 of the column 102 cause the transfer of the ‘shaded’ and ‘unshaded’ states of those respective tristate devices 78 into storage in the computer. The number of tristate devices 78 showing a ‘shaded’ state represents the height ‘H’ of the top of the ball above the playing surface.

[0087] The computer thereupon compares the number of ‘shaded’ states signals received from the tristate devices 78 with the number of ‘shaded’ states stored in the computer which represent the ball height reference level 18, and repeats the process as the ball moves progressively in front of each successive sensor column 106 to the left of the column 100.

[0088] The position of the first sensor column at which the computer finds equality of the detected and reference ‘shaded’ states represents the position of ball touch-down on the playing surface.

[0089]FIG. 4 shows a tennis ball that has touched down at a position to the left of the service reference line 12A and then squashed back on to the service line 12, so making the serve a valid one under the rules mentioned above.

[0090] To allow for squash-back, the computer 84 determines, from the positions of the highest ‘shaded’ sensor 46 in each of two horizonally-spaced sensor columns, the angle 96 at which the ball approaches the playing surface 10, and in accordance with the magnitude of that angle determines empirically the adjustment to be made to the late touch-down position of the ball to arrive at a judgement as to whether that serve is permissible or invalid.

[0091] Although it is possible to reduce the number of sensors in each of the columns to a much smaller figure disposed above and below the ball height reference line, the determination of the angle of ball approach is rendered more accurate by having a large number of sensors in each of the columns.

[0092] The position of ball touch-down may also be determined by causing the computer to scan in the memory 90 (or in the storage means 58) and in the direction of ball flight, the sensor states (as supplied by the tristate devices 78) of corresponding sensors 46 in a given row, and then repeating such scanning progressively in successively lower rows of sensors to find the row in which a sequence of ‘shaded’ sensors is interrupted momentarily by one or only a few ‘unshaded’ sensors.

[0093] A further method of determining the position of ball touch-down comprises scanning in the memory 90 (or in the storage means 58), horizonally (in effect) and in the direction of ball flight, the sensors of the row defining the ball height reference level 18, and determining the column in which a ‘shaded’ sensor is followed by an ‘unshaded’ sensor.

[0094] Another method of determining the position of ball touch-down comprises scanning in the memory 90 (or in the storage means 58), vertically (in effect) successive columns of sensors in the direction of ball flight, to find that column in which a change from a ‘shaded’ sensor to an ‘unshaded’ sensor occurs at the ball diameter reference height.

[0095] Other methods of determining in the memory 90 (or in the storage means 58) the position of ball touch-down may be used instead.

[0096] An alarm device 96 is preferably provided for providing an audible alarm when the computer 84 has judged a service to be invalid.

[0097] It will be noted that in respect of the sensor columns currently sensing the front half of the ball in flight, the bistable and tristate devices 72 and 78 take no part in the determination of ball touch-down position. Only the output signals of the amplifiers 62 and the resultant trigger signals 80 emitted by the associated OR gates 68 are necessary to that determination.

[0098] It will be appreciated that in the case where the ball bounces before the service line 12,28, the ball can be rising off the playing surface as it comes within the radiation field (represented by the area of the sensor array 44). Also, that due to the flattening of the ball on contact with the playing surface, the top of the ball may be sensed in the first instance as being below the ball diameter reference height, and rising towards that height. This circumstance can be prevented from giving a false indication when the actual and reference heights then equate, by ensuring (for example) that the comparison of the height of the top of the ball with the reference height is made only when the height of the top of the ball is sensed as falling towards the reference height. Alternatively, a finding of equality of the detected and reference heights can be negated where the detected height is found to be rising, instead of falling.

[0099] In a modified arrangement of the circuitry described above, determination of the ball touch-down position is determined earlier by the following means: in each successive ‘triggered’ sensor column in turn the height of the uppermost shaded photo-diode is detected (for example, in the manner described above), and transmitted directly to the computer (without prior storage in the storage means 58) for immediate comparison with the game ball diameter reference height (using the computer interrupt method) which has already been stored in the computer memory, to determine whether the height, of the top of the game ball has fallen to that reference height, and hence whether the ball has touched down.

[0100] In a further modification of the circuitry described above, where ‘real-time’ comparisons of the detected and reference heights of the top of the ball are to be used instead, in each of the bistable circuit modules 60 the bistable device 72 is omitted altogether, and the output circuit 70 of the amplifier 62 is connected directly to the input circuit (at 76) of the tristate device 78. In this case, trigger signals 80 delivered on the OR gate output circuits 74 are transmitted directly to the computer 84 to cause it to ascertain (from the relevant tristate devices 78) the height of the top of the ball at the relevant sensor column disposed one half a ball diameter to the rear, and then to compare that detected height with the ball diameter reference height stored in the computer and to determine the position of ball touch-down when the detected and reference ball heights are equal.

[0101] Whereas in the above described system the game ball reference height has been determined by scanning the shadow cast by the ball when lying on the playing surface at the reference line, other methods of determining or establishing the stored game ball reference height may be used.

[0102] In the above described system the front of the ball has been tracked and the height of the top of the ball determined at a position disposed one half of the ball diameter rearwardly of the front of the ball. In an alternative arrangement the back of the ball is tracked and the height of the ball determined at a position disposed one half of the ball diameter forwards of the back of the ball.

[0103] Whilst in the above-described embodiments the radiation has comprised visible light, for various reasons it may be preferable in practice to use infra-red radiation. Other suitable forms of radiation may be used instead if desired. 

1. A method of monitoring in a ball game the position, relative to a reference line extending linearly between near and far ends thereof, of the first contact of a game ball with a playing surface carrying said reference line, which method includes the following steps: (a) from said near end of the reference line directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of a game ball through the radiation when in play near the reference line; (b) at said far end of the reference line sensing the game ball's presence in the radiation by detecting the shadow cast at that end by the game ball; (c) when the game ball is in flight through said field detecting from said shadow the passage of the front of the game ball through successive closely-spaced positions in the direction of flight of the game ball; (d) for each such closely-spaced position, determining the height of the top of the game ball at a position spaced one half the diameter of the game ball rearwardly of the front of the game ball; (e) for each such closely-spaced position, comparing the detected height of the top of the game ball with a stored game ball diameter reference height stored in a storage means; and (f) determining as the position of first contact of the game ball with the playing surface the game ball position at which the reducing height of the top of the game ball has become equal to the stored game ball diameter reference height.
 2. A method according to claim 1, wherein the stored game ball diameter reference height is determined by detecting the height of said shadow when the game ball lies stationary on the playing surface at the reference line.
 3. A method according to claim 1 or 2, which method includes the following steps: (a) from said near end of the reference line directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of the game ball through the radiation when in play near the reference line; (b) at the far end of the reference line, receiving at closely spaced locations in said field respective elements of said radiation, which locations are disposed uniformly in vertical column and horizontal row formation; (c) converting the respective received radiation elements into corresponding electrical signals; (d) storing the respective electrical signals in respective signal storage elements of a storage means for retention therein until set by the passage of the game ball through the radiation, each said storage element when storing a said electrical signal representing a radiation element that has not been interrupted by the game ball, and each said storage element when not so storing a said electrical signal representing a radiation element which has been interrupted by the game ball; (e) defining in said storage means a game ball diameter reference height; (f) tracking in said storage means the front of the game ball as it moves forwardly through the radiation, by following the first shading by the game ball of sensors in successive next columns of sensors in the direction of ball flight; (g) for each of successive positions of the front of the game ball, determining in said storage means the height of the top of the game ball at a position disposed one half of the diameter of the game ball rearwardly from the front of the ball—by determining in the column of sensors at said rearwardly disposed position the height of the uppermost sensor not receiving radiation; (h) determining the instant at which the reducing detected height of the top of the ball equates to the game ball diameter reference height; and (i) determining the position of the game ball at that instant, which position is the position of first contact of the game ball with the playing surface relative to said reference line.
 4. A method according to claim 3, wherein said game ball diameter reference height is defined by determining the lowest row of sensors in which all of the sensors in the row continue to receive radiation when the game ball lies stationary on the playing surface at the reference line.
 5. A method according to claim 3 or 4, wherein in step (f) the shading of at least one sensor in a next vertical column of sensors (referred to hereinafter as a ‘trigger column’) in the direction of ball flight causes the transmission of a staticising signal for retaining in step (g) (until reset) the status of the respective sensor signals in a vertical column of sensors (referred to hereinafter as a ‘controlled column’) disposed rearwardly a distance corresponding to one half of the game ball diameter.
 6. A method according to claim 5, wherein the position of first contact of the game ball with the playing surface relative to said reference line is determined in said storage means by comparing the game ball reference height as determined in step (e) with successive detected heights of the top of the game ball as determined in step (g).
 7. A method according to claim 3 or 4, wherein the position of first contact of the game ball with the playing surface relative to said reference line is determined in said storage means, directly or indirectly, by scanning along each row of sensors in the direction of game ball flight in turn starting from the top row to find the row in which a sequence of sensors not receiving radiation is interrupted temporarily by one or more sensors receiving radiation.
 8. A method according to claim 3 or 4, wherein step (h) comprises scanning in the storage means, directly or indirectly, the respective sensor states (‘shaded’ or ‘unshaded’) of corresponding sensors comprising a given row, and repeating the scanning progressively in successively lower rows of sensors to find the row in which a sequence of ‘shaded’ sensors is interrupted temporarily by one or only a few ‘unshaded’ sensors.
 9. A method according to claim 3 or 4, wherein step (h) comprises scanning horizontally, in the storage means, directly or indirectly, and in the direction of ball flight, the respective storage states (‘shaded’ or ‘unshaded’) of respective sensors of the row of sensors defining the game ball height reference level, and determining in that row the sensor column at which a ‘shaded sensor’ status is followed by an ‘unshaded sensor’ status.
 10. A method according to claim 3 or 4, wherein step (h) comprises scanning vertically, in the storage means, directly or indirectly, and in the direction of ball flight, successive columns of sensors to find that column in which a change from a ‘shaded’ sensor to an ‘unshaded’ sensor occurs at the game ball diameter reference height.
 11. A method according to any one of the claims 3 to 9, wherein the detected position of first contact of the game ball with the playing surface is adjusted, in order to allow for squash-back of the game ball, in dependence upon the angle relative to the playing surface at which the game ball approaches the playing surface.
 12. A method according to any one of the claims 3 to 7, wherein the radiation comprises visible light, or infra-red radiation.
 13. Apparatus for monitoring in a ball game the position, relative to a reference line extending linearly between near and far ends thereof, of the first contact of a game ball with a playing surface carrying the reference line, which apparatus comprises: (a) at said near end of the reference line, radiation emitting means for directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of the game ball through the radiation when in play near the reference line; (b) at said far end of the reference line, radiation receiving means comprising a plurality of radiation sensors for receiving at closely spaced locations in said field respective elements of said radiation, which locations are disposed uniformly in vertical column and horizontal row formation, which radiation receiving means is arranged to sense the game ball's presence in the radiation by the shadow cast at said far end by the game ball; (c) means for detecting when the game ball is in flight through said field the passage of the front of the game ball through successive closely-spaced positions in the direction of flight of the game ball; and (d) means for determining, each such closely-spaced position, the height of the top of the game ball at a position spaced one half the diameter of the game ball rearwardly of the front of the game ball; (e) means for comparing, for each such closely-spaced position, the detected height of the top of the game ball with a stored game ball diameter reference height; and (f) means for determining as the position of first contact of the game ball with the playing surface the game ball position at which the reducing detected height of the top of the game ball has become equal to the stored game ball diameter reference height.
 14. Apparatus according to claim 13, including means for determining from said shadow when the game ball lies stationary on the playing surface at the reference line said game ball diameter reference height for retention in a storage means.
 15. Apparatus according to claim 13 or 14, which apparatus comprises: (a) at said near end of the reference line, radiation emitting means for directing a collimated radiation along and parallel with the reference line to irradiate a predetermined field at said far end of the reference line and lying adjacent the reference line, said field being such as will be materially but not wholly shaded by the passage of the game ball through the radiation when in play near the reference line; (b) at the far end of the reference line, radiation receiving means comprising a plurality of radiation sensors for receiving at closely spaced locations in said field respective elements of said radiation, which locations are disposed uniformly in vertical column and horizontal row formation; (c) converting means for converting the respective received radiation elements into corresponding electrical signals; (d) electrical signal storage means for storing the respective electrical signals in respective signal storage elements of the storage means for retention therein until set by the passage of the game ball through the radiation, each said storage element when storing a said electrical signal representing a radiation element that has not been interrupted by the game ball, and each said storage element when not so storing a said electrical signal representing a radiation element which has been interrupted by the game ball; (e) means for determining as a game ball diameter reference height, the lowest row of sensors in which all sensors in the row continue to receive radiation when the game ball lies stationary on the reference line; (f) means for tracking the front of the game ball as it moves forwardly through the radiation, by reference to the first shading by the game ball of sensors in successive next columns of sensors; (g) means for determining for each of successive positions of the front of the game ball the height of the top of the game ball at a position disposed one half of the diameter of the game ball rearwardly from the front of the game ball—by determining in the column of sensors at said rearwardly disposed position the height of the uppermost sensor not receiving radiation; (h) means for determining the instant at which the reducing detected height of the top of the game ball equates to the game ball diameter reference height; and (i) means for determining the position of the game ball at that instant, which position is the position of first contact of the game ball with the playing surface relative to said reference line.
 16. Apparatus according to claim 15, including for use in step (f) means responsive to the shading of at least one sensor in a next vertical column of sensors (referred to hereinafter as a ‘trigger column’) in the direction of ball flight for transmitting a staticising signal for retaining (until reset) the status of the sensor signals generated in a vertical column of sensors (referred to hereinafter as a ‘controlled column’) disposed rearwardly a distance corresponding to one half of the game ball diameter.
 17. Apparatus according to claim 16, wherein said means for determining the position of first contact of the game ball with the playing surface relative to said reference line comprises means for comparing with the said game ball reference height successive sets of said sensor signals retained in respective controlled columns.
 18. Apparatus according to claim 15, wherein said means for determining the position of first contact of the game ball with the playing surface relative to said reference line comprises means for scanning, in said storage means, directly or indirectly, and in the direction of game ball flight, along each row of sensors in turn starting from the top row to find the row in which a sequence of sensors not receiving radiation is interrupted temporarily by one or only a few sensors receiving radiation.
 19. Apparatus according to claim 15, including means for scanning ‘horizontally’, in the storage means, directly or indirectly, and in the direction of ball flight, the respective sensor states (‘shaded’ or ‘unshaded’) of respective sensors of the row of sensors defining the game ball height reference level, and determining in that row the sensor column at which a ‘shaded sensor’ status is followed by an ‘unshaded sensor’ status.
 20. Apparatus according to claim 15, including means for scanning ‘vertically’ in the storage means, directly or indirectly, successive columns of sensors in the direction of ball flight, to find that column in which a change from a ‘shaded’ sensor to an ‘unshaded’ sensor occurs at the game ball diameter reference height.
 21. Apparatus according to any one of the claims 15 to 20, including means for measuring the angle relative to the playing surface at which the game ball approaches that surface, and means for adjusting the detected position of first contact of the game ball with the playing surface, in order to allow for ‘squash-back’ of the game ball, in dependence upon the said angle relative to the playing surface.
 22. Apparatus according to any one of the claims 13 to 21, wherein the radiation comprises visible light, or infra-red radiation.
 23. A method according to any one of the claims 1 to 12, substantially as hereinbefore described with reference to and as illustrated by the accompanying diagrammatic drawings.
 24. Apparatus according to any one of the claims 13 to 22, substantially as hereinbefore described with reference to and as illustrated by the accompanying diagrammatic drawings. 